1. Field of the Invention
The present invention relates generally to a semiconductor device and a mask pattern, and more particular to processing of a capacitor of a semiconductor device.
2. Description of the Related Art
In recent years, there has been a demand for reduction in power consumption and enhancement in operation speed of LSIs (Large Scale Integrated Circuits), and it has become important to add a nonvolatile function to a memory. In addition, introduction of new materials is indispensable in order to enhance the performance and increase the number of functions. Various novel materials have been incorporated in modern devices. For example, special attention has been paid to a nonvolatile ferroelectric memory (ferroelectric RAM) in which a ferroelectric thin film is used as an inter-electrode insulation film of a capacitor, and the development of the nonvolatile ferroelectric memory is proceeding rapidly.
Such novel materials include a relatively large number of materials that contain an element, which diffuses in a fabrication step, or an element, which easily reacts with some other material. A typical composite oxide, which is used as a material of a ferroelectric film, is PZT (Pb(Zr, Ti)O3). Lead (Pb) and titanium (Ti), which are contained in PZT, are instances of such elements.
There may be a case where a part of elements that constitute an underlayer film diffuses and reacts in a PZT film, which is an amorphous film, in a crystallizing heat treatment step, leading to degradation in ferroelectricity or in leak current characteristic. In addition, constituent elements in the amorphous film may diffuse into the underlayer film in the crystallizing heat treatment step, and constituent elements of the PZT crystal film may become defective at an interface between the PZT film and the underlayer film. Furthermore, at an initial stage of the crystallizing heat treatment, a constituent element in the amorphous film may react with a constituent element in the underlayer film, and the film quality of the underlayer electrode itself may not merely deteriorate and increase the resistance thereof, but also the crystallinity of the underlayer film may deteriorate. Consequently, the crystal growth of the amorphous film may be hindered. As a result, these factors considerably degrade the reliability in ferroelectricity and electrical characteristic of the PZT film.
Moreover, in the heat treatment step for the ferroelectric film, high temperatures of, e.g. 600 to 700° C., have widely been used. The electrode film itself needs to withstand such high temperatures from the standpoint of both electrical and structural aspects. In short, the electrode film is required to have a low resistance and a high melting point. These conditional constraints pose a difficult problem in technical development of the fabrication process. The same drawbacks, as mentioned above, may occur not only in the method of forming an ordinary ferroelectric capacitor, wherein a film is first grown at low temperatures and then the film is subjected to high-temperature heat treatment to form a crystalline insulating film, but also at a time of forming a ferroelectric film while crystallizing the film or at a time of performing heat treatment after formation of a ferroelectric capacitor.
Since the capacitor is an important component for the operation of the memory, the thermal/chemical stability of the capacitor is to be sought from the above-mentioned standpoint. However, conversely speaking, to seek the thermal/chemical stability makes it difficult to carry out capacitor processing that makes use of reactivity such as RIE (Reactive Ion Etching). A major processing technology in the existing fabrication process is dry etching. A halogen-based compound is typically used as a reactive gas (or etching gas or etchant gas) etchant for the dry etching, thereby performing physical/chemical processing.
In the processing technology for the ferroelectric memory, however, the melting points/boiling points of the constituent elements of the electrode film and the halogen-based compound of the etchant are very high. Thus, it is very difficult to execute etching unless the physical etching factors are enhanced by assisting with additional energy, for example, by heating the substrate or applying a high bias.
In many cases, use is made of a method of enhancing factors of physical etching by using Ar as a main reactive gas (or main etching gas or main etchant gas) main etchant. In such cases, etching damage occurs and selectivity to a resist lowers. Moreover, as illustrated in FIG. 6, such a serious problem arises that an etched matter 72 re-adheres to sidewalls of a capacitor 71. The re-adhered residual matter is called “fence”, which may lead to an increase in leak current, a decrease in breakdown voltage, degradation in reducing ferroelectric material due to degradation in step coverage of a cover film such as Al2O3, and a decrease in surface planarity in a Chemical Mechanical Polish (CMP) step that is executed after deposition of an interlayer insulation film. In the meantime, a ferroelectric film of, e.g. PZT is very susceptible to acids, and its ferroelectricity deteriorates due to moisture. Thus, the ferroelectric film is not suited to processing by wet etching.